19 . Difference between getCurrentSession() and openSession() in Hibernate ?

getCurrentSession() :
Obtains the current session. The "current session" refers to a Hibernate Session bound by Hibernate behind the scenes, to the transaction scope.
A Session is opened when getCurrentSession() is called for the first time and closed when the transaction ends. It is also flushed automatically before the transaction commits. You can call getCurrentSession() as often and anywhere you want as long as the transaction runs. Only the Session that you obtained with sf.getCurrentSession() is flushed and closed automatically.

openSession() :
If you decide to use manage the Session yourself the go for sf.openSession() , you have to flush() and close() it.
It does not flush and close() automatically.
Example :
Transaction tx =session.berginTransaction();

Session session = factory.openSession();

try {
tx.begin();

// Do some work
session.createQuery(...);
session.persist(...);

session.flush(); // Extra work you need to do

tx.commit();
}
catch (RuntimeException e) {
tx.rollback();
throw e; // or display error message
}
finally {
session.close(); // Extra work you need to do
}

20.What are the types of Hibernate instance states ?
Three types of instance states:

• Transient -The instance is not associated with any persistence context
• Persistent -The instance is associated with a persistence context
• Detached -The instance was associated with a persistence context which has been closed – currently not associated

21.What are the types of inheritance models in Hibernate?
There are three types of inheritance models in Hibernate:

• Table per class hierarchy
• Table per subclass
•  Table per concrete class

22.What is Hibernate Query Language (HQL)?
Hibernate Query Language is query language which is used to develop the data independent query language in the application. This HQL queries are not related to any database. Hibernate offers a query language that embodies a very powerful and flexible mechanism to query, store, update, and retrieve objects from a database. This language, the Hibernate query Language (HQL), is an object-oriented extension to SQL.

23.What are the ways to express joins in HQL?
HQL provides four ways of expressing (inner and outer) joins:-

• An implicit association join
• An ordinary join in the FROM clause
• A fetch join in the FROM clause.
• A theta-style join in the WHERE clause.

24 . Transaction with plain JDBC in Hibernate ?

If you don't have JTA and don't want to deploy it along with your application, you will usually have to fall back to JDBC transaction demarcation. Instead of calling the JDBC API you better use Hibernate's Transaction and the built-in session-per-request functionality:

To enable the thread-bound strategy in your Hibernate configuration:

set hibernate.transaction.factory_class to org.hibernate.transaction.JDBCTransactionFactory
set hibernate.current_session_context_class to thread

Session session = factory.openSession();
Transaction tx = null;
try {
tx = session.beginTransaction();

// Do some work
session.load(...);
session.persist(...);

tx.commit(); // Flush happens automatically
}
catch (RuntimeException e) {
tx.rollback();
throw e; // or display error message
}
finally {
session.close();
}